MONTREUX, Switzerland, Jan 23 (Reuters) - The United Nations hopes that political talks between the warring sides in Syria will clinch local ceasefires to allow vital food and medicines to reach millions of civilians, the U.N. humanitarian chief said on Thursday.

Valerie Amos, in an interview with Reuters a day after an international conference on Syria, said mediator Lakhdar Brahimi would press the government and opposition on these humanitarian issues at meetings due to start later in the day.

The two sides, meeting for the first time, vented their mutual hostility on Wednesday but Brahimi said the enemies may be ready to discuss prisoner swaps, local ceasefires and humanitarian aid.

"I have discussed this with Mr. Brahimi and he'll continue to push this. Because political negotiations can take a very long time. And as we saw yesterday, there are big differences between the parties," Amos said in an interview in Montreux, Switzerland.

"But if we are able to get a major push on getting into these communities, it would make a significant difference."

She said it was crucial to gain access to some 250,000 people trapped in besieged communities, many of them in Aleppo, Homs and near Damascus, who have been out of reach for many months. Some say they have been reduced to eating grass in order to avoid starvation.

Another 2.5 million people are in "hard-to-reach" areas, having received U.N. relief supplies just once or so, Amos said.

"The key issue for us is that control of communities shifts all the time. We want to really take advantage when there is a sense we can move very quickly to try to do that," she said, adding that she had met with an opposition delegate in Montreux.

"MODEST PROGRESS"

Amos, who submitted a confidential written brief to the U.N. Security Council last Friday, said that there had been little improvement since world powers called unanimously in October for both sides to grant greater access for aid workers and convoys.

"I indicated to the Security Council, as I have done before, that we have made some modest progress on administrative hurdles that we have faced, things like visas for staff and arrangements in place in terms of clearance (for convoys)," Amos said.

"But on the really big-ticket items, like protection of civilians, demilitarisation of schools and hospitals, access to besieged and hard-to-reach communities, there has been little or no progress at all," she said.

A food aid delivery reached Yarmouk, a suburb of Damascus besieged by forces of President Bashar al-Assad, on Jan. 18, badly needed after months of isolation but not enough for thousands of trapped civilians who are malnourished and without medicine.

Some 50 of 400 planned U.N. food parcels were delivered in the Yarmouk Palestinian district at the time, Amos said, calling it a "tiny bit of progress".

Russia - which is Syria's main arms supplier - has been a "strong advocate" in nudging Syrian authorities to grant visas for U.N. aid workers, she said.

But administrative measures are not enough and there were only three U.N. aid convoys in December, after the government rejected three and safety guarantees were not in place for two others, she said. Nine convoys delivered goods in November.

Amos said that she would brief the Security Council in person in the second half of February but would not say whether she would seek a tough resolution. Russia has blocked previous resolutions it considered too harsh on its ally Assad.

"I will be saying to the Council: look, access remains a major issue...But I think we also have to brainstorm, given since October we have made such little progress on these big- ticket items," she said.

U.N. aid officials were looking at options for delivering more aid and would discuss them with influential countries at their next high-level meeting set for Feb. 3 in Rome, she said.

"I hope there will be conclusions out of that that I could put to the Security Council," she said of the talks, to be hosted by Italian Foreign Minister Emma Bonino.

Amos, asked whether humanitarian corridors were under consideration, said that they have proved "notoriously difficult" in war zones as they usually require enforcement.

"But there are truces, pauses, and reconciliation talks going on in some areas locally," she said.

"What can we do on the back of that and what is coming out of these political talks which we can come on the back of ... we will be looking at a range of different measures." (Reporting by Stephanie Nebehay; Editing by Mark Trevelyan)